1. Field of the Invention
The present invention relates to an image read device in use with a copying machine or a facsimile, for example, and more particularly improvements of an optical system of the image read device which guides light reflected by an original document to an image sensor CCD.
2. Discussion of the Prior Art
An image read device has been used for a copying machine, a facsimile, or the like. The image read device, as shown in FIG. 1A, is made up of a first carriage 4 which moves at velocity "v" along a platen glass 1 and includes a light source 2 and a first mirror element 3, a second carriage 7 which moves at velocity (1/2)v (i.e., 1/2 the velocity of the first carriage 4), and includes a second mirror element 5 and a third mirror element 6, a lens 8 for forming on an image sensor CCD 9 an image with light reflected by the third mirror element 6, and the image sensor CCD 9 for converting the image formed thereon into electrical signals.
To read a picture on an original document 10 located on the platen glass 1, the carriages 4 and 7 are moved while the original 10 is irradiated with light from the light source 2. Light reflected by the surface of the original 10 is guided through an optical system including the first, second and third mirror elements 3, 5 and 6 to the lens 8 which in turn forms an image on the CCD 9.
Usually, a stepping motor is used as a drive source in the scanning system of the image read device. The higher harmonic components of the exciting signal for the motor causes a vibration of the motor. The harmonic components are transferred to the carriages. As shown in FIG. 7, a mirror M is mounted on the carriage, while being supported at both ends by a mirror positioning means 11. With the construction, when the carriage is vibrated, the mirror M vibrates in the direction of the illustrated arrows, so that the mirror is warped as indicated by dotted lines. When the mirror M is warped, in the case of the first mirror element 3, the read position is shifted by .DELTA.x in the vertical or slow scan direction as shown in FIG. 3, and hence the read image is also shifted. In the case of the second and third mirror elements 5 and 6, the warp of each mirror element brings about a change of the reflected light direction as shown in FIG. 2A. This distorts the read image by .DELTA.y.
Many measures for removing the image shift and distortion caused by the vibration of the scanning system have been proposed. For example, one measure is an anti-vibration means for reducing vibration energy to be transferred to the mirror. Another measure is a damping means for damping vibration energy transferred to the mirror. A further means is to set the natural frequency of the mirror element to be different from the fundamental frequency and a higher harmonic frequency of the stepping motor. For details of the measures, reference is made to Japanese Patent Laid-Open Publication No. Sho. 2-166880 and Japanese Patent Laid-Open Publication No. Sho. 63-312772, for example.
When the first mirror element 3 is vibrated and warped by the higher harmonic component of the stepping motor, the actual read position is shifted from the correct read position when the mirror is warped. The shift of the read position is observed near the center of the span of the main or fast scan direction, and depends on the period and amplitude of the vibration. When the read position is shifted, the registration of colors is lost in the slow scan direction in the case Of the color image read device for reading color information through a plurality of scans.
When the second and third mirror elements 5 and 6 are vibrated and warped, vertical lines lose their linearity in both side areas as viewed in the fast scan direction (this phenomenon is called a vertical-line distortion). The vertical-line distortion varies a little in level and is observed over the entire range as viewed in the slow scan direction when the stepping motor, which is for driving the scan system, runs in low and medium speed regions. When it runs at a high speed, the vertical-line distortion gradually decreases from its original level. Particularly, in the color image reader, the vertical-line distortion results in the incorrect registration of colors in the fast scan direction.
The optical image reading system of the image read device must be installed in a limited space. In this respect, it is desirable that the mirror vibration killing means is simple in construction.